Before play testing the Mythic system I talked to my most experienced gamer friends about it and these were the thoughts we had.
Progression through trials is an interesting and novel way of progressing the through mythic tiers, allowing progression to be based more on story elements. As you increase in tiers though you need to fulfill many more trials, making each tier take longer to get than the previous, therefore tiers will likely not stay in step with every other level. Also the trials felt a lot like achievements and players are likely to start treating them that way. For example: during an easy fight an archmage may ask the rest of the group to drag the fight on so (s)he can cast a spell from each different school.
The mythic spells each seem to have about a 3 level adjustment modification to them, which is really good for balancing. And if a play or GM wants a mythic spell other than the ones listed, then it is easy to just pick the spell you want and manually modify it by 3 levels.
I really liked that each class seems to be able to fit into more than one mythic paths. That should help accommodate different play styles.
The flaws were an interesting idea and I think will be fun since each character will need to keep track of their flaw and how it can be used against them. A bad GM could also start throwing lots of the characters weakness at them because it is their flaw, but a bad GM will be a bad GM with or without the mythic system.
The only major concern I had was that mythic abilities are so much more powerful than regular abilities that this system seems inappropriate for regular level campaigns. I think the mythic system would be well balanced for epic (level 21+) campaigns though. The only modifications I would make for turning mythic tiers into epic levels would be to have the hit points from each tier also modified by constitution modifiers, add a +1 on attacks at every odd tier, a +1 on all saves at every even tier, and some additional skill ranks for each tier depending on the mythic path.
Overall, I think the mythic system is pretty well balanced for very high levels, but would be unbalancing for most games. If my group wants to try to mythic system I will post the results of the play test.